1. Field of the Invention
This invention relates to a focus detecting device for cameras or the like.
2. Description of the Related Art
There is a known focus detecting device which sets a plurality of focus detection areas in a photograph image plane, detects the focus in each of the focus areas, and determines finally one defocus amount on the basis of this plurality of defocus amounts. As examples of determining one defocus amount, there are the following methods.
1. Closest focusing priority method: a method by which the amount of defocus at the closest side among the defocus amounts is chosen. PA1 2. Contrast priority method: a method by which the amount of the defocus in a focus detection area which has the highest contrast and reliability among a plurality of focus detection areas is selected. PA1 3. Statistical average method: a method by which the final amount of defocus according to the statistical average of a plurality of defocus amounts is determined.
Such focus detecting devices of the prior art have the following problems.
1 Problems of the closest priority method PA0 2 Problems of the contrast priority method PA0 3 Problems of the statistical average method
Focusing on a closest object may not be consistent with the photographer's intention. For example, when a photographer takes aim at an animal inside a wire cage as an object, the wire cage is focused on, which does not satisfy his intentions. When the contrast of a focus detection area which is selected as the closest is low and the defocus amount varies each time a focus point is detected, the final amount of the defocus is not stable since it varies in response to the defocus amount.
The contrasts of two focus detection areas may be substantially the same maxium but the defocus amounts may differ greatly. When the focus detection area of the maximum contrast alternates each time a focus is detected, shifting occurs between two defocus amounts and stability is poor. If there is even one focus detection area with a high contrast, an object of a focus detection area with a high contrast is focused on regardless of the photographer's intentions. For example, when the face of a human with a low contrast is viewed against a background with a high contrast, the background is focused and the intentions of the photographer are not satisfied. Further, in a focus detection area with the highest contrast, when the contrast is not very high, the defocus amounts vary each time a focus is detected, and there is no stability since the final amount of the defocus varies.
Averaging defocus amounts for a plurality of areas may result in focusing that is inappropriate. For example, when a human in the foreground is viewed against a distant background, neither the far view nor the human is focused.
To sum up the drawbacks of the prior art as described above, when one area is selected from a plurality of divided focus detection areas and the amount of the defocus of the area is adopted as a final amount of defocus, there is a certain probability that an object which a photographer views is focused, but this probability is not sufficient. Also, since the variations in the amount of the defocus are reflected in variations of the final defocus amount, the stability is poor.
When the average amount of the defocus of a plurality of divided focus detection areas is adopted as the final defocus amount, variation in the final amount of the defocus may be small and the stability may be improved. However, the probability that the object at which the photographer takes aim will be in focus is small.